The present invention relates to a system for controlling injection of fuel for an internal combustion engine with a direct fuel injection system, and more particularly to the system for controlling a fuel injection pulse width and fuel pressure and hence quantity of the fuel to be injected in accordance with operating conditions of the engine.
As the simplest method of producing scavenge air in a two-stroke cycle engine, a crankcase scavenging system is generally used. In an ordinary crankcase-scavenged two-stroke engine, vacuum is produced in an enclosed crankcase of the engine to induce fresh air when a piston ascends. When the piston descends, burned gas is discharged through an exhaust port while the fresh air in the crankcase is admitted into a cylinder, thereby scavenging the cylinder. However, at light engine load, since intake air quantity is small, the scavenging becomes insufficient, resulting in misfire and hence in an irregular engine operation. In addition, torque characteristics corresponding to the engine operating conditions becomes unstable, thereby consuming much oil. Fuel escapes without burning so that fuel consumption deteriorates and the amount of noxious exhaust gases increases. Moreover, at a heavy engine load, volume of the crankcase is insufficient for scavenging.
In order to solve such a problem, only air is delivered to the cylinder and fuel is injected directly to the cylinder by an injector after the exhaust port is closed. Such a direct fuel injection system may be a type where only high pressurized fuel is injected or a type where low pressurized fuel mixed with air is injected. In order to provide a charge stratification, it is preferable to inject only the fuel by high pressure, since the fuel can be injected in a short time and the injection timing can be set near the ignition timing, thereby restraining the diffusion of the fuel.
The fuel injection must have a large dynamic range so as to accurately inject a predetermined quantity of the fuel within a predetermined period at the high pressure. In addition, the injector must bear high pressure and temperature of the burned gas.
Japanese Patent Application Laid-Open 60-40756 discloses a fuel injection system for a four-stroke cycle engine having a simple relief valve and a pressure sensor provided instead of a pressure regulator. A quantity of fuel to be injected is quickly controlled in accordance with the change in fuel pressure by adjusting the fuel injection elapse time in dependency on the output signal of the fuel pressure sensor. However, since the fuel pressure is not controlled in the system, the dynamic range of the injector cannot be increased.
Japanese patent Application Laid-Open 61-272447 discloses a system where the fuel pressure which is the difference between the pressure of fuel supplied to the injector and the pressure in the intake passage is controlled so as to converge to a desired value. Several levels of the desired value are provided so that the quantity of injected fuel corresponding to the injection elapse time can be linearly controlled. However, since the fuel pressure is controlled in accordance with the intake pressure, the system is provided for injecting fuel at a relatively low pressure. Therefore, a large increase of the linearly controlled range can not be expected. Namely, assuming that the injection elapse time is constant, the quantity of the fuel injected from the injector is proportional to the value of the square root of the fuel pressure. Since the absolute value of the fuel pressure is inherently low in the fuel injection system, the change in the quantity of the injected fuel is small, even if the desired value for the fuel pressure is changed.